Those Tesla/Panasonic cells are simply more affordable.
According to UBS analyst Colin Langan, the teardown of lithium-ion battery cells produced by Panasonic/Tesla, LG Chem, Samsung SDI and Contemporary Amperex Technology (CATL) revealed that Panasonic/Tesla cells are some 20% more cost efficient than the next best offer (LG Chem).
It's estimated that the Panasonic 21700 (2170) type cylindrical lithium-ion cells cost $111/kWh, which is $37 less than LG Chem ($148/kWh). If the other manufacturers are at even higher prices, the advantage is tremendous, although we are not sure how much cheaper is to make modules/packs with other types of cells (the overall difference on a pack level could be smaller).
Anyways, 75 kWh of batteries should cost Tesla $8,325 or $11,100 in the case of 100 kWh - before they start building the pack, as we understand. In the case of LG Chem, it would be $2,800 or $3,700 more respectively.
UBS expects that the four mentioned manufacturers will control 70% of the EV battery market by 2025 and that costs will fall by 10% within 2-3 years.